Early risers are often glorified — there’s the well-known saying about the early bird catching the worm, and many articles, blog posts, and Reddit threads about the benefits of getting up at 4:30 a.m.

But if rising early is so good for us, why don’t all humans get up at the crack of dawn — or at least try to? Why do some of us consider ourselves morning people (also known as larks) or night owls?

The answer is simple: Everyone has a natural internal clock — a chronotype or circadian rhythm — that determines when they’re most alert and most sleepy. That means that we’re not all meant to be morning people. As chronobiologist Till Roenneberg explains in his book, “Internal Time: Chronotypes, Social Jet Lag, and Why You’re So Tired,” the internal factors determining when we like to wake up are primarily genetic. The biggest external factor is exposure to light, especially sunlight.

Research into chronotypes helps explain why people have natural sleep patterns, how that changes throughout life, the differences in circadian rhythms between men and women, and what we can do to modify our schedules.

What’s my chronotype?

Most of us think of ourselves as morning or night people, but those divisions aren’t scientific — they’re just ways of comparing ourselves to one another.

“Where you define owl or lark is really arbitrary,” says Dr. David Welsh, an associate professor studying circadian clocks at UC San Diego. Welsh says that if you look at large surveys of populations, you get a normal distribution of chronotypes — most people have fairly “average” chronotypes, some prefer to get up a bit earlier or later, and small groups naturally rise extremely early or late. There’s no line that distinguishes different chronotypes.

Your chronotype is more than a feeling — it can be verified by measuring your body temperature and the levels of certain hormones in your blood at different times during the day, since those factors are regulated by our circadian clocks.

According to most research on the topic, we have a genetic predisposition for a certain chronotype, but the genetic factors that cause some people to feel alert and productive earlier versus later are complex. A number of different genes are involved, many of which are still being discovered.

The broad range of human chronotypes also take several other variables into account. For most of their lives, men have slightly later chronotypes than women. And alongside preferences about when we go to bed, people also require different amounts of sleep. Most adults need between seven and nine hours, though some require slightly more or less, and a tiny number need far more or far less.

Most of our internal clocks also don’t match the 24-hour cycle of our planet. Instead, they tend to be a little longer than 24 hours (though some people’s clocks are far longer or shorter). Those whose internal clocks run short tend to be early risers — sleep pressure builds up for them more quickly, pushing them to fall asleep earlier and rise with the sun.

Roenneberg explains in his book that there might be an evolutionary explanation for why people have such varied body clocks. As humanity evolved, it was probably advantageous for some of us to be more alert at different times of day. That would have allowed members of groups to stay awake at night in order to watch for predators or hunt for food that was easier to catch in the dark.

The changes our internal clocks undergo throughout our lifetimes could also be related to this evolutionary legacy. Chronotypes start early for young children, become later in adolescence, then get earlier again as we age. Roenneberg notes that the shift toward a later schedule among adolescents tends to coincide with the time of life at which humans peak in athletic ability — around 20. The most physically capable people are therefore also most comfortable staying awake through the night. (In general, night owls can keep working longer in tests that require staying awake for long periods of time.)

Today, however, the observed chronotype changes that happen in teenagers have led many sleep researchers to suggest that high schools should simply start later.

How do we reset the clock?

Body clocks can be changed — when someone moves from Los Angeles to London, they eventually shift their schedule, after all — but only to a certain degree. So how much can we adjust our internal clock?

“You can’t wish yourself to be a morning person if you’re really an evening person, but what you can do is alter your light exposure,” Welsh says.

Daylight naturally helps us feel alert, while darkness prompts the body to produce hormones that make us sleepy. That’s how we adjust to new time zones.

According to Welsh, those who want to start waking up earlier should try to expose themselves to bright light first thing in the morning for at least half an hour, since early sunlight exposure acts a daily reset. (In experiments where people are kept away from clocks and natural light, internal clocks can run wild.) Maximizing the amount of natural light you get during the rest of the day and avoiding light at night will also help the brain and body adjust to a new schedule. To move your body clock in the later direction (for travel, for example, or if early risers are trying to sleep in later), Welsh recommends exposing yourself to bright light at night.

In a lab, Welsh says, it’s usually possible to reset someone’s body clock by carefully controlling light exposure, but it can be tough to create that kind of light exposure schedule in the real world. Plus, certain people simply find it easier to adjust their chronotypes than others do.

In some cases, it can be impossible to totally change our natural inclinations, which is why some people struggle with the daily schedule that work and school impose. The very fact that most people use alarms to wake up means that for the most part, our days begin before we’re biologically awake, according to Roenneberg’s book. Either that, or we’re simply not getting enough sleep.